This application claims the priority of Japanese Patent Application No. 2001-375644 filed on Dec. 10, 2001 which is incorporated herein by reference.
The present invention relates to a method for controlling the engagement of frictionally engaging elements provided in a power transmission, through which engagement control, the rotational output of an engine is transmitted to wheels with some adjustment.
Generally, an automobile includes a construction that transmits the driving force of an engine through a power transmission to wheels for driving the vehicle body, and for controlling this power transmission, a plurality of frictionally engaging elements such as friction clutches are typically incorporated in the transmission. For optimizing the engagement characteristic of the friction clutches in engagement control, it is important to monitor accurately the engagement control value at the time of the initiation of engagement action. Generally, the power transmissions used in an automobile is equipped with hydraulically actuated friction clutches. In this case, hydraulic actuation pressures are evacuated from the clutches completely for disengagement. Therefore, when any friction clutch is to be engaged, the friction clutch must be at first cleared of the ineffective or invalid part of the stroke of the clutch piston before an actual engagement of the clutch starts. This arrangement of the hydraulic clutch has an inherent problem of experiencing a delay prior to the actual engaging actuation. To solve this problem, it is important to determine the pressure that initiates the real engaging action of the friction clutch in engagement control or the pressure that completes the clearing of the ineffective part of the stroke of the clutch and sets the clutch into pre-engagement condition, and to utilize this determined engagement-control pressure for controlling the engagement actuation of the friction clutches.
Especially in a case where a friction clutch is used as a starting clutch in a vehicle, if the starting clutch, which has been relieved of the pressure used for an engagement actuation, is supplied again with oil to generate the pressure necessary for controlling the starting of the vehicle, there is a delay before the actual engaging action of the clutch occurs. When the pressure in the clutch increases, the part of the stroke of the piston which does not contribute to the real engagement of the clutch is cleared (hereinafter referred to as xe2x80x9cinvalid stroke clearingxe2x80x9d). After the completion of the invalid stroke clearing, when the starting clutch begins an actual engaging action, the pressure actuating the engaging action may become substantially large and make the clutch to engage abruptly, causing a problem of startup shock. If such an abrupt engagement occurs in the starting clutch, then the load on the engine will increase suddenly and may cause an engine stall.
Furthermore, for the purpose of improving fuel economy and reducing air pollution, many vehicles nowadays are equipped with an idling elimination control, in which the operation of the engine is stopped when the vehicle comes into a halt. In such a vehicle, when the vehicle starts, the engine must be started with the starting clutch being controlled to engage appropriately. Therefore, it is important to determine the pressure that sets the starting clutch into a pre-engagement condition, i.e., a condition where the starting clutch can start a real engaging action.
On this background, a control that learns and sets an optimal pressure for a frictionally engaging element such as a starting clutch has been developed and set in practice to achieve the above mentioned pre-engagement condition or an engagement-starting condition. For example, there is a method for learning the pressure at which the starting clutch begins the real engaging action by detecting a change in the load on the engine experienced when the transmission is shifted from a neutral position (N range) to a drive position (for example, D range) from a change in the negative pressure of the suction of the engine. Furthermore, in a vehicle whose fuel supply to the engine is controlled to stop while the vehicle is in deceleration, another method is applied for learning the pressure at which the starting clutch begins the real engaging action. This method is based on the fact that, in this vehicle, while the fuel supply is stopped during the deceleration, the slip factor of the clutch is controlled such that the torque-transmission capacity of the clutch calculated from the engagement-control pressure of the clutch equals the friction torque of the engine.
However, in a case where the pressure at which the starting clutch begins the real engaging action is determined based on the detection of a load change during a shift from the N range to the D range, there is a problem that if the rotational speed of the engine changes during the determination, then the determination cannot be performed accurately. This limitation, i.e., the determination must be carried out while the rotational speed of the engine is kept constant, makes the determination of the engagement-control pressure difficult to perform. While the fluctuation of the engine rotational speed is minimized, the engagement-control pressure of the clutch is changed slightly, and the transmission is switched between the drive ranges. By repeating the switching of the drive ranges, optimal pressure values are measured. However, the measurement performed in this way takes a relatively long period of time (for example, a few minutes). If the measurement takes a longer period of time, then it is more difficult to prevent a fluctuation in the rotational speed of the engine.
In a case where the pressure at which the starting clutch begins the real engaging action is measured on the basis of the engagement-control pressure of the clutch, whose slip factor is controlled while the vehicle is in deceleration with the termination of the fuel supply to the engine, it is necessary to decelerate the vehicle for the measurement to be performed. Therefore, the measurement in this case is a large scale operation and takes a long period of time.
As mentioned above, for a vehicle which is controlled to eliminate engine idling, it is important to determine the engagement-control pressure that sets the clutch into a real engaging action in the engagement control of the clutch at the starting of the vehicle. In this case, unless the vehicle is actually driven for the control system to learn the engagement-control pressure, the system sets the vehicle not to perform an idling elimination control. Therefore, before the vehicle is shipped out of a factory, the engagement-control pressure at which the clutch begins the real engaging action should be measured and determined by a test on a bench using the above mentioned method. However, this measurement operation takes a long period of time and is difficult to perform accurately as mentioned above. As a result, this operation increases the manufacturing cost of the vehicle. As there is a choice that such setting operation is not performed at the shipping of the vehicle, if it is not performed at the factory, then the idling elimination control of the vehicle is not performed until the engagement-control pressure is learned and set appropriately in real driving situations. This condition affects adversely the fuel economy of the vehicle and may cause the driver or owner to feel something wrong with the vehicle.
It is an object of the present invention to provide a method for measuring simply and quickly an engagement-control value when the frictionally engaging element (friction clutch) begins an engaging action or when it is set into a pre-engagement condition where it can start immediately an engaging action.
To achieve this objective, the present invention provides a method for controlling the engagement of a frictionally engaging element (for example, the starting clutch 5 described in the following embodiment) in a power transmission (for example, the continuously variable transmission CVT described in the following embodiment), through which engagement control, the rotational output of an engine is adjusted and transmitted to drive wheels. In this method, at least either the rotational speed of the engine or a parameter which corresponds to the rotational speed of the engine is measured while the frictionally engaging element is being actuated from a disengaged condition to an engaged condition, with the engine being operated under a constant operational condition. When there is a change in at least any of the rotational speed of the engine and the parameter being measured, an engagement-control value is calculated for the frictionally engaging element, and the engagement-control value calculated is stored as an initial control value used for starting the engagement of the frictionally engaging element, so that the initial control value is used to control the next engagement session of the frictionally engaging element.
By this method, while the operational condition of the engine is kept constant, only the engaging condition of the frictionally engaging element is changed to measure the initial control value at which the engaging action of the frictionally engaging element starts. This measurement can be performed simply and quickly (in about 10 seconds), so it can be performed simply, for example, at the end of a manufacturing line before the vehicle is shipped from a factory, without complicating a pre-shipment inspection. If the initial control value is set by this measurement in pre-shipment condition in this way, then the vehicle equipped with an idling elimination control can perform the idling elimination control without any problem immediately after the delivery of the vehicle. Therefore, the method according to the present invention can improve the product value of the vehicle.
The initial control value set in this way is used for the engagement control of the frictionally engaging element when the power transmission is operated for the first time. Thereafter, the engagement-control value at which the frictionally engaging element begins an engaging action is calculated from the engagement characteristic determined from the condition of the frictionally engaging element being controlled to engage in real driving situations. Preferably, the engagement-control value is calculated and used to renew the initial control value in memory, continually. In this way, the engagement control can be performed appropriately and accurately in correspondence to the real driving situation even if the frictionally engaging element undergoes deterioration with age.
In this method, the above mentioned parameter may be the suction negative pressure of the engine, and when the suction negative pressure changes to a higher load side and when the rotational speed of the engine decreases, the engagement-control value for the frictionally engaging element is stored as the initial control value.
On the other hand, although the suction negative pressure of the engine changes to a higher load side, if the rotational speed of the engine increases or does not change, then this condition is judged as a case of wrong learning, so the engagement-control value for the frictionally engaging element in this case is not stored as the initial control value.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.